The present invention is related in general to the field of semiconductor devices and processes and more specifically to the design and fabrication of a land-grid array/ball-grid array package based on flip-chip assembly generally of micromechanical devices and specifically of digital mirror devices.
Micromechanical devices include actuators, motors, sensors, spatial light modulators (SLM), digital micromirror devices or deformable mirror devices (DMD), and others. The technical potential of these devices is especially evident when the devices are integrated with semiconductor circuitry using the miniaturization capability of semiconductor technology.
SLMs are transducers that modulate incident light in a special pattern pursuant to an electrical or other input. The incident light may be modulated in phase, intensity, polarization or direction. SLMs of the deformable mirror class include micromechanical arrays of electronically addressable mirror elements or pixels, which are selectively movable or deformable. Each mirror element is movable in response to an electrical input to an integrated addressing circuit formed monolithically with the addressable mirror elements in a common substrate. Incident light is modulated in direction and/or phase by reflection from each element.
As set forth in greater detail in commonly assigned U.S. Pat. No. 5,061,049, issued on Oct. 29, 1991 (Hornbeck, xe2x80x9cSpatial Light Modulator and Methodxe2x80x9d), deformable mirror SLMs are often referred to as DMDs in three general categories: elastometric, membrane, and beam. The latter category includes torsion beam DMDs, cantilever beam DMDs, and flexure beam DMDS. Each movable mirror element of all three types of beam DMD includes a relatively thick metal reflector supported in a normal, undeflected position by an integral, relatively thin metal beam. In the normal position, the reflector is spaced from a substrate-supported, underlying control electrode, which may have a voltage selectively impressed thereon by the addressing circuit.
When the control electrode carries an appropriate voltage, the reflector is electrostatically attracted thereto and moves or is deflected out of the normal position toward the control electrode and the substrate. Such movement or deflection of the reflector causes deformation of its supporting beam storing therein potential energy which tends to return the reflector to its normal position when the control electrode is de-energized. The deformation of a cantilever beam comprises bending about an axis normal to the beam""s axis. The deformation of a torsion beam comprises deformation by twisting about an axis parallel to the beam""s axis. The deformation of a flexure beam, which is a relatively long cantilever beam connected to the reflector by a relatively short torsion beam, comprises both types of deformation, permitting the reflector to move in piston-like fashion.
A typical DMD includes an array of numerous pixels, the reflectors of each of which are selectively positioned to reflect or not to reflect light to a desired site. In order to avoid an accidental engagement of a reflector and its control electrode, a landing electrode may be added for each reflector. It has been found, though, that a deflected reflector will sometimes stick or adhere to its landing electrode. It has been postulated that such sticking is caused by intermolecular attraction between the reflector and the landing electrode or by high surface energy substances adsorbed on the surface of the landing electrode and/or on the portion of the reflector which contacts the landing electrode. Substances which may impart such high surface energy to the reflector-landing electrode interface include water vapor or other ambient gases (e.g., carbon monoxide, carbon dioxide, oxygen, nitrogen) and gases and organic components resulting from or left behind following production of the DMD. A suitable DMD package is disclosed in commonly assigned U.S. Pat. No. 5,293,511 issued on Mar. 8, 1994 (Poradish et al., xe2x80x9cPackage for a Semiconductor Devicexe2x80x9d).
Sticking of the reflector to the landing electrode has been overcome by applying selected numbers, durations, shapes and magnitudes of voltage pulses to the control electrode. Detail can be found in U.S. Pat. No. 5,096,279, issued on Mar. 17, 1992 (Hornbeck et al., xe2x80x9cSpatial Light Modulator and Methodxe2x80x9d). Further improvement of the sticking problem is disclosed in commonly assigned U.S. Pat. No. 5,331,454, issued on Jul. 19, 1994 (Hornbeck, xe2x80x9cLow Reset Voltage Process for DMDxe2x80x9d). This patent describes a technique for passivating or lubricating the portion of the landing electrode engaged by the deformed reflector, and/or the portion of the deformed reflector which engages the landing electrode. Passivation is effected by lowering the surface energy of the landing electrode and/or the reflector, which is, in turn, effected by chemically vapor-depositing on the engageable surfaces a monolayer of a long-chain aliphatic halogenated polar compound, such as perfluoroalkyl acid. Objects do not easily, if at all, stick or adhere to low energy surfaces, which are also usually expected to be resistant to sorption thereonto of high surface-energy imparting substances such as water vapor.
Refinements of the passivation method are disclosed in U.S. Pat. No. 5,939,785, issued on Aug. 17, 1999 (Klonis et al., xe2x80x9cMicromechanical Device including Time-release Passivantxe2x80x9d), and U.S. Pat. No. 5,936,758, issued on Aug. 10, 1999 (Fisher et al., xe2x80x9cMethod of Passivating a Micromechanical Device within a Hermetic Packagexe2x80x9d). The method an enclosed source time-releasing a passivant, preferably a molecular sieve or binder impregnated with the passivant. Further, the method is placing a predetermined quantity of the passivant in the package just after device activation, and is then immediately welding a hermetic lid (free of passivant during the welding process) to the package.
The described sensitivity of most micromechanical devices would make it most desirable to protect them against dust, particles, gases, moisture and other environmental influences during all process steps involved in device assembly and packaging. It is, therefore, especially unfortunate that conventional assembly using gold wire bonding does not permit the removal of any protective material from the micromechanical devices after wire bonding completion, so that the devices have to stay unprotected through these process steps. As a consequence, yield loss is almost unavoidable.
Furthermore, today""s overall package structure for micromechanical devices, based on multi-level metallization ceramic materials, and method of fabrication is expensive. This fact conflicts strongly with the market requirements for many applications of micromechanical devices, which put a premium at low device cost and, therefore, low package cost.
An urgent need has therefore arisen for a coherent, low-cost method of encapsulating micromechanical chips and for a low cost reliable package structure. The structure should be flexible enough to be applied for different micromechanical product families and a wide spectrum of design and process variations. Preferably, these innovations should be accomplished while shortening production cycle time and increasing throughput.
According to the present invention, a low-cost ceramic package, in land-grid array or ball-grid array configuration, for micromechanical components is fabricated by coating the whole integrated circuits wafer with a protective material, selectively etching the coating for solder ball attachment, singulating the chips, flip-chip assembling a chip onto the opening of a ceramic substrate, underfilling the gaps between the solder joints with a polymeric encapsulant, removing the protective material form the components, and attaching a lid to the substrate for sealing the package.
The package structure disclosed is flexible with regard to solder and underfill materials and geometrical detail such as storage space for chemical compounds within the enclosed cavity of the package.
It is an aspect of the present invention to be applicable to a variety of different semiconductor micromechanical devices, for instance actuators, motors, sensors, spatial light modulators, and deformable mirror devices. In all applications, the invention achieves technical advantages as well as significant cost reduction and yield increase.
In a key embodiment of the invention, the micromechanical components are micromirrors for a digital mirror device. In this case, the lid is a plate made of glass or any other material transparent to light, and the protective material is a photoresist as used in photolithographic processes.
It is another aspect of the present invention to keep the sensitive micromechanical components safely protected until the final attachment of the lid, resulting in significantly higher assembly and process yield and enhanced device quality and reliability.
Another aspect of the invention is to use well-controlled solder ball attachment and solder joint underfill processes, providing a package with low mechanical stress and necessary control for providing planarity requirements.
Another aspect of the invention is to be applicable to single-level metal ceramic substrates, which can be manufactured at low cost.
Another aspect of the invention is to provide assembly and packaging designs and processes with flexibility to produce land-grid array or ball-grid array packages.
These aspects have been achieved by the teachings of the invention concerning structure and methods suitable for mass production.
The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.